The present invention relates to ultrasonic sensors for measuring a physical properties of materials within a defined space, and relates more particularly to a time gate ultrasonic sensor.
Various ultrasonic distance measuring and level measuring systems are known. For example, Ellinger et al. U.S. Pat. No. 4,815,323 teaches an "echo ranging" ultrasonic transducer transmitting an ultrasonic signal which is reflected from a liquid surface in an aircraft fuel container and then sensed. The round-trip time from sending to receiving is measured and the fuel quantity and density are computed in a central processing unit as a function of the round trip time and stored data.
In another type of ultrasonic sensor as taught by U.S. Pat. No. 4,299,114 to Silvermetz et al. , an ultrasonic transmission between transmit and receive transducers closes a feedback loop of a circuit which oscillates when feedback increases to a predetermined amount. When a material level in a container rises to a level where the transmit and receive transducers are mounted, the higher amount of feedback through the material causes the circuit to oscillate. This type requires a relatively large difference between the amount of feedback with liquid and the amount of feedback with air for stable operation. Undesired feedback through a sensor body can reduce stability, as well. If air in entrapped in the material, feedback can be reduced and sensing fails. This type of sensor typically is quite large and not suitable for insertion in a container through a small hole such as a 3/4" NPT threaded hole. In this type of sensor, misalignment of the transducers can reduce feedback through the materials so that sensing fails.
Ultrasonic sensors can include additional crystals (transducers) mounted or "piggybacked" on the sensing (receiving) crystals for a "self-test" function. The additional crystals are driven to excite the receiving crystal to complete the feedback loop to test the functioning of the receiving crystal without material in the gap. This does not completely test the ability of the- sensor to sense material in the gap, however, since it does not detect that a crystal has become un-bonded from the sensor body. This arrangement increases the overall size and complexity of the sensor and wiring, as well.
Silvermetz et al U.S. Pat. No. 4,299,114 discloses another self-test configuration in which, during test mode, an ultrasonic system monitors ultrasonic transmission through a support structure between a transmit crystal and a receive crystal. The crystals are connected in an oscillator feedback loop which oscillates if the system is operational. The system is deemed operational if the amplitude of the ultrasonic signals transmitted through the support structure is sufficient to maintain oscillation. However, this configuration is not easily adaptable to operate when the sensor includes a plastic support structure, because the attenuation in plastic weakens feedback and it is difficult to sustain oscillation.